This invention relates to wire braiding.
In the manufacture of a variety of products, such as shielded electrical cable and flexible hydraulic and pneumatic hosing, wire is tightly woven over a tubular core to form a braided sheath. The wire strands are typically wound on supply bobbins which are driven along sinuous paths in opposing directions around the tubing, passing over and under one another to lay the wire in an interlaced woven pattern over the surface of the tubing. Each supply bobbin is mounted on a carrier which pays out the wire under controlled tension during braiding.
The wire is often wound on the bobbins, and payed out, in multi-strand bundles. Since the wire strands within each bundle are not of precisely the same length, the shorter strands are placed under greater tension during the braiding operation and stretch slightly until the lengths of the strands are equalized. When textile filaments or "soft" wire is braided, considerable elongation under tension is permissible, and tension alone works well to compensate for variations in length among the bundled strands.
However, in the construction of certain products, such as hydraulic hose, it is essential to use "hard" wires having great tensile strength. It is the strength of braided-wire sheath which permits the hose to handle high pressure without bursting. Because hard wire can be stretched very little, even under great tension, braiding tension alone does not adequately eliminate length variations, and the consequent poor distribution of load among the bundled strands significantly reduces the pressure-handling capability of the braided sheath.
It is accordingly the principal object of the present invention to increase the strength and endurance of braided-wire sheath.
In accordance with the invention, the distribution of tension among the bundled strands of high-tensile-strength wire is continually equalized during braiding by wrapping the slack strands, which are under lesser tension, about the remaining strands which are under greater tension.
In accordance with the invention, the longer wires under less tension are positioned at the outside of the twisted grouping where they are helically wound around the shorter wires which are under greater tension. A preferred braiding pattern employs adjacent pairs of bundles woven in a two-over, two-under weave. By twisting the adjacent bundles in each of the pairs in opposite directions, one clockwise and the other counterclockwise, the parallel combination is balanced and any tendency to produce a net twisting force to the tubing covered by the braided sheath is eliminated.
A more detailed description of the method and apparatus for weaving the braided-wire sheathing contemplated by the present invention is presented in the aforementioned U.S. Pat. No. 4,092,897, the disclosure of which is incorporated herein by reference and briefly summarized below.